Combination ISE and solutions therefor

ABSTRACT

Described is a combination, PVC-membrane, Ion-Selective Electrode (ISE) with a replaceable sensing module and a single-junction, sleeve-type reference electrode. pH and ion-selective electrodes (ISEs) in the &#34;Combination&#34; configuration, i.e., single probes containing both the sensing and reference half-cells, are more convenient, more economical, and can be used to analyze smaller samples than an equivalent pair of separate half-cell electrodes.

This application is a continuation of application Ser. No. 08/609,225filed on Mar. 1, 1996 now U.S. Pat. No. 5,830,338.

BACKGROUND OF THE INVENTION

A complete potentiometric measurement cell consists of two half-cells(see FIG. 1 attached hereto). Ion-selective electrode measurement cellsare potentiometric cells where one-half cell is the ion-selectiveelectrode (ISE) and the other is the reference electrode. The referenceelectrode usually includes the following: an internal reference elementcomposed of a metal or metal-salt combination; one or two referenceelectrolyte solutions (salt bridge solutions); and one or two liquidjunctions, which are orifices, porous barriers, or other restrictions.One junction acts to prevent excessive mixing of a reference electrolytewith the test solution (sample), but at the same time provideselectrical contact between that reference electrolyte and sample. Thisreference electrolyte may also directly contact the reference element,or may contact via a second liquid junction a second referenceelectrolyte. In this latter case, the second reference electrolytecontacts the reference element. ISEs and reference electrodes can beconstructed as separate assemblies which are then used together in atest solution to comprise an entire measuring cell. Often, however, allof these elements are combined in a single mechanical assembly or probeand referred to as a combination ISE. Combination ISEs are advantageousin that they are often less expensive than a pair of individualhalf-cell electrodes, and the more compact geometry allows use insmaller volumes of sample.

SUMMARY OF THE INVENTION

The present invention is a combination ISE which has a replaceablesensing membrane module which also forms part of the reference junction,and a single internal reference electrolyte solution compartment. Thebenefits of this invention are threefold. First, the ISE sensing moduleis replaceable; this is beneficial because often the ion-selectivesensing membrane has the shortest useful life of all components. Second,using the sensing module to form part of the reference junction isbeneficial because it permits a preferred reference junctionconfiguration, an annular sleeve, to be used without geometricalcompromise, and locate them both at the very tip of the probe so that aminimum immersion depth is required. Third, having a single liquidjunction and reference electrolyte simplifies the mechanicalconstruction. These concepts are best understood through reference tothe drawings accompanying this specification, namely FIGS. 1 and 2. Itshould be understood that, in use, the bottom ends of these probes, asoriented in the drawings, would be immersed in sample to a typical depthof 1 to 5 cm, and that the probes themselves are typically in the rangeof 12 cm long and 1 cm in diameter.

In this invention, combination probes with PVC sensing membranes weredesigned and evaluated for the following analyte ions: nitrate, nitrite,potassium, calcium, and magnesium. The design goals included thefollowing features:

overall performance characteristics equivalent to or better thanhalf-cell electrodes used with separate single- or double-junctionreference electrodes;

easily replaceable sensing membrane module;

single-junction, flowing, flushable, sleeve-type reference junction;

ability to analyze small samples (<1 Ml);

reference filling solution compositions that are non-contaminating evenin small sample volumes;

are optimized so as to minimize the effect of temperature in the analyteconcentration ranges of principal interest:

and are equitransferent, that is, they contain anions and cations withnearly equal mobilities so that liquid junction potentials are minimal.

In one especially preferred embodiment, the combination ISE of thepresent invention comprises in combination a tapered sensing module, theouter surface of which serves as the inner cone of the reference sleeve.A spring-loaded cap assembly provides constant tension between the innerand outer cones of the sleeve which can be flushed by a thumb-press onthe cap. A threaded, hydrophobic, double O-ring seal connects the moduleto the electrode inner body.

Another embodiment of the present invention is directed to a series ofreference electrolyte solutions for use with the preferred ISE, suchthat the following ISE modules can be used with a single referenceelectrolyte and junction: ammonium, barium, bromide, cadmium, calcium,chloride, cupric, cyanide, fluoride, fluoroborate, iodide, lead,lithium, magnesium, nitrate, nitrite, perchlorate, potassium, silver,sulfide, sodium, surfactant, thiocyanate, and water hardness.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a traditional pair of half-cell electrodes. The ISE in thedrawing has a replaceable sensing module. The reference electrode is adouble junction design with a sleeve-type outer junction. Theseelectrodes represent the prior art of the present invention.

FIGS. 2a, 2b and 3 illustrate a preferred embodiment of the combinationion selective electrode (ISE) which has a replaceable sensing membranemodule which also forms part of the reference junction, and a singleinternal electrolyte solution compartment.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

As described above, and as illustrated in FIGS. 2a, 2b and 3, thepresent invention is directed to a combination, PVC-membrane,Ion-Selective Electrode (ISE) with a replaceable sensing module and asingle-junction, sleeve-type reference electrode. pH and ion-selectiveelectrodes (ISEs) in the "Combination" configuration, i.e., singleprobes containing both the sensing and reference half-cells, are moreconvenient, more economical, and can be used to analyze smaller samplesthan an equivalent pair of separate half-cell electrodes.

In this invention, combination probes with PVC sensing membranes weredesigned and evaluated for the following analyte ions: nitrate, nitrite,potassium, calcium, and magnesium.

The preferred combination electrode of the present invention includesthe following features:

(a) overall performance characteristics equivalent to or better thanhalf-cell electrodes used with separate single- or double-junctionreference electrodes;

(b) easily replaceable sensing membrane module;

(c) single-junction, flowing, flushable, sleeve-type reference junction;

ability to analyze small samples (<1 mL); and

(d) reference filling solution compositions that are non-contaminatingeven in small sample volumes, and are optimized so as to minimize theeffect of temperature in the analyte concentration ranges of principalinterest;

and are equitransferent, that is, they contain anions and cations withnearly equal mobilities so that liquid junction potentials are minimal.

The preferred design of the combination electrode of the presentinvention is shown in detail in FIGS. 2a, 2b and 3. As illustrated, thecombination electrode of this invention includes a tapered sensingmodule (10); the outer surface (12) of which serves as the inner cone(14) of the reference sleeve (16). A spring-loaded cap assembly (18)provides constant tension between the inner (14) and outer cones (20) ofthe sleeve which can be flushed by a thumb-press (22) on the cap (24). Athreaded, hydrophobic, double O-ring seal (26) connects the module tothe electrode inner body.

FIG. 1 shows the prior art, i.e., a pair of half-cell electrodes. TheISE (100) in FIG. 1 has a replaceable sensing module (102). Thereference electrode (104) is a double junction design with a sleeve-typeouter junction (106). The sleeve-type junction (106) is preferred amongmany types of reference junctions because of superior performancecharacteristics.

The combination ISE in FIGS. 2a, 2b and 3 combines the desirablefeatures of each of the prior art individual half-cells of FIG. 1, intoa single probe with single liquid junction. The present invention is inparticular, directed to the preferred means by which these individualfeatures are incorporated into a single, functional probe. One desirablefeature of the sensing half-cell is the replaceability of the sensingmembrane module. There are several desirable features of the referencehalf-cell, including:

(1) The reference half-cell has a sleeve-type outer junction, that is,one which is formed from an inner body which is rod-shaped with thebottom end flared into the shape of a cone. The inner body residesconcentrically in an outer sleeve, which is a hollow cylinder with wallthickness sufficient to allow the inside of the bottom end to bechamfered to form a surface which mates with the cone of the inner bodyin the same manner as a conventional tapered glass joint. A liquidjunction in the form of an annulus is thereby created between thereference electrolyte and sample solution, and the rate of flow ofreference electrolyte through this junction out into the sample underthe influence of gravity is controlled by the angles of the matingsurfaces of the annulus and the amount of force which maintains them incontact. This force is imparted by a spring located in the electrodecap. The spring pushes the outer sleeve against the conical end of theinner body.

(2) The junction is situated at the very end of the probe body,permitting contact with the sample through immersion of the probe to aminimal depth, allowing use in small sample volumes.

(3) The reference electrolyte can be flushed through the junction bypressing the spring-loaded cap while holding the outer sleeve fixed.This action forces the inner body downward relative to the sleeve asoriented in the drawing, thereby widening the annulus and allowinggreater flow.

(4) The reference electrolyte can be replenished through a hole locatednear the top of the outer sleeve.

(5) The reference electrolyte does not interfere with operation of thesensing membrane, although it does flow into the sample at a finiterate, allowing species it contains to come in contact with the membrane.In the reference half-cell shown in FIG. 1, contamination is avoided bymeans of a double-junction design, a concept familiar inelectrochemistry. Certain species must be present in an electrolyte inorder for a stable potential to be established at a reference element.Sometimes these species can interfere with the sensing membrane whenthey flow with the electrolyte into the sample. Therefore, two referenceelectrolytes are used. The inner electrolyte contains those speciesnecessary for establishment of this reference potential. This solutioncommunicates with an outer electrolyte via an inner liquid junction,which could be a small hole or a plug of porous, inert material such asporous ceramic. The outer electrolyte contains species which areeffective in maintaining a stable junction potential at both the innerand outer junctions, but need not contain species which establish thereference element potential and which may also interfere with membranefunction.

In the combination ISE shown in FIGS. 2a 2b and 3, only one electrolyteis used. It contains species which establish a stable reference elementpotential and a stable junction. In order to avoid interference with thesensing membrane when a variety of ion-selective membranes are used, avariety of internal reference electrolytes are used. Preferredelectrolytes include those shown on Table 1;

                  TABLE 1                                                         ______________________________________                                        Summary of Orion Optimum Results Reference Filling Solutions                  ISEs                                                                          ______________________________________                                        Cd.sub.2+                                                                           1.7 M KNO.sub.3                                                                            equitransferent RFS for the specified S/J                  CA.sup.2+                                                                           0.64 M KCl   combination ISEs resulting in small                        F     0.06 M NaCl  temperature coefficients and insignificant                       sat, w/AgCl  contaminations to the sample.                                    Triton X-100                                                                  0.002%                                                                  Pb.sup.2+                                                                           1 M KNO.sub.3                                                                              nearly equitransferent RFS for the                         Ag/S.sup.2-                                                                         0.02 M KCl   specified S/J combination ISEs with                        Cl.sup.-                                                                            sat. w/AgCl  the optimum composition for small                          CN.sup.-                                                                            Triton X-100 0.01%                                                                         temperature coefficients, small                                               contaminations to the samples, the                                            electrode surfaces and the junction.                       Ag.sup.+ /S.sup.2-                                                                  1 M KNO.sub.3                                                                              nearly equitransferent RFS for the                               0.001 M AgCl specified S/J/ combination ISE with                                           minimum temperature coefficient.                           Cu.sup.2+                                                                           1 M KNO3     nearly equitransferent, silver free RFS for                Br.sup.-                                                                            0.001 M AgCl the specified S/J combination IDEs with                    I.sup.-            minimum temperature coefficients and                                          contaminations.                                            K.sup.+                                                                             1 M NaCl     RFS for the specified S/J combination ISE                        sat. w/AgCl  with optimum composition for small                                            temperature coefficient, small                                                contamination, and stable junction                                            potential.                                                 NO.sub.3.sup.-                                                                      2 M (NH.sub.4).sub.2 SO.sub.4                                                              RFS for the specified S/J combination                      NO.sup.2.sub.-                                                                      0.001 M NaCl ISEs for reduced temperature coefficients,                       sat. w/AgCl  small sample contaminations, and stable                                       junction potential.                                        ______________________________________                                    

The present invention has been described in detail, including thepreferred embodiments thereof. However, it will be appreciated thatthose skilled in the art, upon consideration of the present disclosure,may make modifications and/or improvements on this invention and stillbe within the scope and spirit of this invention as set forth in thefollowing claims.

What is claimed is:
 1. A combination ion selective electrode (ISE) comprising a replaceable sensing membrane module which also forms part of the reference junction, and a single internal electrolyte solution compartment.
 2. The combination ISE of claim 1, wherein the outer surface of the replaceable sensing module serves as the inner cone of a reference junction sleeve.
 3. The combination ISE of claim 2, wherein the reference junction configuration is an annular sleeve.
 4. A combination ion selective electrode (ISE) comprising in combination:(a) a reference half-cell which includes outer junction sleeve, formed from an inner body which is rod shaped with the bottom end flared into the shape of a cone; (b) wherein said inner body resides concentrically in an outer sleeve which is a hollow cylinder with a wall thickness sufficient to allow the inside of the bottom end to be chamfered to form a surface which mates with the cone of the inner body; (c) thereby creating a liquid junction in the form of an annulus between the reference electrolyte and the sample solution, and wherein the rate of flow of reference electrolyte through this junction out into the sample under the influence of gravity is controlled by the angles of the acting surfaces of the annulus and the amount of force which maintains them in contact.
 5. The combination ISE of claim 4, further comprising an electrode cap at the top of said hollow cylinder, said cap including spring means for pushing the outer sleeve against the conical end of the inner body.
 6. The combination ISE of claim 4, further comprising only one reference electrolyte.
 7. The combination ISE of claim 6, wherein the reference electrolyte contains species necessary for establishment of a stable reference potential.
 8. The combination ISE of claim 7, wherein the reference electrolyte contains species which are effective in maintaining a stable junction potential.
 9. The combination ISE of claim 8, wherein the reference electrolyte is an aqueous solution consisting essentially of 1.7 M KNO₃, 0.64 M KCl, 0.06 M NaCl, saturated with AgCl and containing Triton X-100 at 0.002% by weight.
 10. The combination ISE of claim 8, wherein the reference electrolyte is an aqueous solution consisting essentially of 1 M KNO₃, 0.02 M KCl, saturated with AgCl and containing Triton X-100 at 0.002% by weight.
 11. The combination ISE of claim 8, wherein the reference electrolyte is an aqueous solution consisting essentially of 1 M KNO₃ and 0.001 M AgCl.
 12. The combination ISE of claim 8, wherein the reference electrolyte is an aqueous solution consisting essentially of 1 M KNO₃ and 0.001 M NaCl.
 13. The combination ISE of claim 8, wherein the reference electrolyte is an aqueous solution consisting essentially of 1 M NaCl saturated with AgCl.
 14. The combination ISE of claim 8, wherein the reference electrolyte is an aqueous solution consisting essentially of 2 M (NH₄)₂ SO₄, 0.001 M NaCl, saturated with AgCl. 